Recording materials for the production of offset printing plates (also known as “pre-sensitized printing plates”) are usually supplied in stacks of 20 units or more. Extended storage times, the action of pressure and/or elevated ambient temperatures frequently result in the plates adhering to one another. On removal of individual plates from the stack, scratches may then form on the front and/or back. The problem of undesired adhesion can be substantially eliminated with the aid of separating paper. The paper is particularly necessary in the case of recording materials having an aluminium support with an uncoated back. However, the separating paper results in new problems. The recording materials are frequently produced in in-line finishing plants, in which the plates are automatically cut to the desired size and packed. The separating paper is likewise inserted automatically. However, this step is relatively slow and susceptible to faults. In addition, the paper in some cases affects the radiation-sensitive layer and adversely changes its properties. This may result in discoloration of the layer, due to a change in the pH, a drop in its light sensitivity or rapid ageing. With surface-sealed papers, the interaction between paper and radiation-sensitive layer can be reduced; however, such papers are significantly more expensive. In relatively large print shops, the plate stacks provided with separating paper are generally processed in automatic plants, with the paper usually being blown out. This operation is again relatively slow and susceptible to faults. In addition, the paper cannot be recycled and has to be disposed of.
The recording material described in JP-A 02/040657 manages without separating paper. A UV-cured layer produced from a photopolymerizable material is located on the back of its aluminium support. In addition to monomers, the composition used for the production of the back coating may also comprise photosensitizers, binders, fillers, inhibitors for preventing thermally induced polymerization of the monomers and other additives.
JP-A 06/202312 discloses a recording material for the production of offset printing plates whose aluminium support is likewise coated on the back with an organic polymer, such as polyethylene, polypropylene, polybutadiene, polyester, polycarbonate, polyvinyl acetal, polyvinyl chloride, polystyrene or a methacrylate resin. The back coating reduces attack by the aqueous-alkaline developer on the aluminium support. The light-sensitive layer in this recording material comprises from 1 to 10% by weight of a compound which is insoluble in the developer.
A recording material having an anodized aluminium support, a photopolymerizable layer on the aluminium oxide layer produced by anodization, and a back coating having a thickness of from 0.1 to 8.0 μm is disclosed in JP-A 09/265176. This back coating consists of a saturated copolymerized polyester resin, a phenoxy resin, a polyvinyl acetal or a vinylidene chloride copolymer, each of which has a glass transition temperature Tg of 20° C. or above. This is intended to prevent scratching of the plates during transport in the stack and delamination of the radiation-sensitive layer due to excessive adhesion to the back of the overlying plate.
A recording material for the production of offset printing plates which can be stacked without separating paper is also described in EP-A 528 395. It comprises a support (made of aluminium), a layer of an organic polymeric material having a glass transition temperature of not less than 20° C. with a thickness of from 0.01 to 8.0 μm on the back of the support, and a light-sensitive layer on the front of the support. A discontinuous matting layer consisting of particles having a mean diameter of not greater than 100 μm and a mean height of not greater than 10 μm is in turn located on the light-sensitive layer. However, matting layers, in particular those comprising a material having a low glass transition temperature, tend to stick to the back of the overlying plate in the stack. This may cause relatively large areas of the radiation-sensitive layer to be delaminated, meaning that the recording material can then no longer be used further.
A recording material having a matting layer on the radiation-sensitive layer and a further, likewise discontinuous matting layer on the back of the support material is disclosed in EP-A 883 028. The support material is generally an aluminium web having a length of up to several thousand meters and a thickness of about 0.2 mm which is generally rolled up again after the coating (“coil-to-coil process”). A higher pressure prevails in the interior of the roll than in the outer regions. This results in the matting layer located on the front being damaged to a non-uniformly great extent. A recording material having a greatly damaged matting layer requires longer evacuation times later in the contact copying frame. This problem is substantially solved by the additional matting layer on the back.
EP-A 490 515 relates to a pre-sensitized printing plate which, after imagewise exposure, is developed using an aqueous alkali metal silicate solution. In order to prevent the developer from dissolving aluminium out of the back of the plate, this is provided with an organic polymeric coating which is insoluble in the developer. The coating comprises polymers such as polyethylene, polypropylene, polybutene, polybutadiene, polyamide, polyurethane, polyurea, polyimide, polysiloxane, polycarbonate, epoxy resins, polyvinyl chloride, polyvinylidene chloride or polystyrene. It may also comprise a thermally or photochemically curing component.
DE-A 199 08 529 describes a recording material having a support which has on the back a layer comprising an organic polymeric material having a glass transition temperature of 45° C. or above, and a pigmented light-sensitive layer located on the front of the support.
DE-A 100 29 157 relates to a recording material for the production of offset printing plates which has a dimensionally stable support, a radiation-sensitive layer located on the front of the support, and a layer which comprises an organic polymeric material and is resistant to processing chemicals located on the back of the support, where the glass transition temperature of the organic polymeric material is 35° C. or above, and the layer located on the back is pigmented.
The known recording materials with a back coating have the disadvantage that their production is complex and is frequently associated with technical problems. On use of solid particles (for example silica gel pigments), two-phase systems inevitably form. The solid constituents here can sediment again very quickly, meaning that it is frequently difficult to apply a two-phase mixture of this type in the form of a layer. In addition, the solid particles are not always compatible with the surrounding matrix material. In addition, silica gel particles may be attacked by very strong developers (i.e. those having a pH of greater than 13) and can be dissolved under certain conditions, for example after extended exposure. This then again results in problems associated with the dissolution of oxide on the back, with corrosion phenomena in the case of silver plates or even detachment of the back layer.